A switching converter is used to step up or step down a DC voltage or convert a DC voltage into an AC voltage and vice versa. Various types of switching converters are available. Among these, there is a constant current-controlled switching converter for detecting a current flowing into a load and/or detecting a current flowing through a switching element of the switching converter and stabilizing the detected current to a predetermined value through a feedback control.
FIG. 1 is a circuit diagram of a flyback type switching converter. A switching converter 100r, to which an input voltage VIN from a power supply (not shown) is input, generates and supplies an output voltage VOUT to a load 502 by stepping down the input voltage VN while stabilizing a current ILED (referred to as a load current or a driving current) flowing through the load 502 to a target current value TREF. For example, the load 502 is a light emitting diode (LED) string and the switching converter 100r sets the load current ILED to the target current value TREF based on the target brightness of the LED string.
The switching converter 100r includes an output circuit 102 and a control circuit 200r. The output circuit 102 includes a smoothing capacitor C1, a rectifying diode D1, a switching transistor M1, a primary coil LP, an auxiliary inductor LZT and a detection resistor RCS.
In an ON period of the switching transistor M1, a current flowing through the switching transistor M1 is flown through the detection resistor RCS. A voltage drop (detection voltage) VCS of the detection resistor RCS is fed back to a current detection (CS) terminal of the control circuit 200r. 
A voltage VZT produced in the auxiliary inductor LZT is divided by resistors and the divided voltage is input to a ZT terminal of the control circuit 200r. 
The control circuit 200r includes a current detection comparator 202, a zero-cross detection circuit 204, a driving logic unit 206 and a driver 208.
FIG. 2 is an operation waveform diagram of the switching converter 100r of FIG. 1.
In an ON period of the switching transistor M1, a current (primary current) IP flows through the primary coil LP of a transformer T1, the switching transistor M1 and the detection resistor RCS. The primary current IP increases with time and the detection voltage VCS rises accordingly. The current detection comparator 202 compares the detection voltage VCS with a target voltage VREF set corresponding to the target current value IREF. The current detection comparator 202 asserts a peak current detection signal S1 (for example, the signal S1 is at a high level) if the detection voltage VCS reaches the target voltage VREF, that is, if the current IP reaches a peak current IPEAK (=VREF/RCS). In the ON period, energy stored in the transformer T1 increases. During the ON period, no current flows through a secondary coil LS of the transformer T1.
The driving logic unit 206 changes a pulse signal S2 to an OFF level (for example, a low level) corresponding to a turn-off operation of the switching transistor M1 when the peak current detection signal S1 is asserted. The driver 208 turns off the switching transistor M1 in response to the pulse signal S2.
In an OFF period of the switching transistor M1, a current (secondary current) IS flown through the secondary coil LS of the transformer T1 decreases with time and a voltage VZT decreases accordingly. The zero-cross detection circuit 204 compares the voltage VZT of the auxiliary inductor LZT with a threshold voltage VZERO set to about zero. Based on the comparison result, the zero-cross detection circuit 204 detects that the secondary current IS is zeroed (zero-cross), and asserts a zero-cross detection signal S3.
The driving logic unit 206 changes the pulse signal S2 to an ON level (for example, a high level) corresponding to a turn-on operation of the switching transistor M1 when the zero-cross detection signal S3 is asserted. The driver 208 turns on the switching transistor M1 in response to the pulse signal S2.
The control circuit 200r repeats the above operation. The load current ILED is a current obtained by smoothing the secondary current IS by means of the smoothing capacitor C1 and is expressed by the following equation (1).ILED=1/2×IPEAK′×TOFF/T  (1)Where, IPEAK′ is a peak of the secondary current IS and T is a switching period (TON+TOFF). Equation (1) may be rewritten into the following equation (2). Here, a forward voltage drop of the rectifying diode D1 is ignored.ILED=IPEAK2×LS/VOUT  (2)
As can be seen from Equation (2), the driving current ILED depends on the output voltage VOUT, that is, a forward voltage of the load 502. That is, the switching converter 100r of FIG. 1 is problematic in that the driving current ILED is varied due to the variation of the forward voltage of the load (e.g., LED string) 502. In addition, it is also problematic that the driving current ILED is affected by a change in the inductance of the secondary coil LS of the transformer T1.
To solve the aforementioned problems, a method of connecting a resistor for detection of the driving current ILED in series to the load 502, feeding back a voltage drop of the resistor to the control circuit 200r, and controlling the driving current ILED to be equal to a target value has been proposed. However, applications requiring electrical isolation between a primary side and a secondary side need a photo coupler or a shunt regulator in order to feedback a detection voltage of the secondary side to the primary side, which may result in increased costs.
The above problems may also occur in a variety of loads other than the LED string. That is, in any load, if its impedance is varied or fluctuated, a voltage drop (the output voltage VOUT) across the load when the same load current flows is changed, which may cause a change in the load current ILED.